The eye lens contains a structural protein, alpha crystallin, composed of two homologous primary gene products alpha A2 and alpha B2. In certain rodents, still another alpha crystallin polypeptide, alpha AIns, occurs, which is identical to alpha A2 except that it contains an insertion peptide between residues 63 and 64. In this paper we describe the complete alpha A crystallin gene that has been cloned from DNA isolated from Syrian golden hamster. Evidence is provided that the alpha A gene is present as a single copy in the hamster genome. The detailed organization of the gene has been established by means of DNA sequence analysis and S1 nuclease mapping, revealing that the gene consists of four exons. The first exon contains the information for the 68 base-pair long 5' non-coding region as well as the coding information for the first 63 amino acids. The second exon encodes the 23 amino acid insertion sequence, the third exon codes for amino acid 87 to 127 of the alpha AIns chain, whereas the last exon encodes the C-terminal 69 amino acids and contains the information for the 523 base-pair long 3' non-coding region. The second exon is bordered by a 3' splice junction (A X G/G X C), which deviates from the consensus for donor splice sites (A X G/G X T). This deviation is found in both hamster and mouse. An internal duplication was detected in the first exon by using a DIAGON-generated matrix for comparison. By means of similar DIAGON-generated matrices it was confirmed that the amino acids coded for by the third and fourth exons are homologous to the small heat-shock proteins of Drosophila, Caenorhabditis and soyabean. The implications of the differential splicing and the evolutionary aspects of the detected homologies are discussed.